Chromatin organization involves DNA wound around histone octamers that form nucleosomes. Core histones with N-terminal tails extending from compact nucleosomal core particles can be acetylated or deacetylated at epsilon lysine residues affecting histone-DNA and histone-non-histone protein interactions. Histone deacetylases (HDACs) catalyze the deacetylation of histone and non-histone proteins and play an important role in epigenetic regulation. There are currently 18 known HDACs that are organized into three classes: class I HDACs (HDAC1, HDAC2, HDAC3, HDAC8 and HDAC11) are mainly localized to the nucleus; class II HDACs (HDAC4, HDAC5, HDAC6, HDAC7, HDAC9 and HDAC10), which shuttle between the nucleus and the cytoplasm; and class III HDACs (SIRT1-7), whose cellular localization includes various organelles.
Class II HDACs are further characterized as class IIa HDACs and class IIb HDACs.
HDAC9 is class IIa histone deacetylase highly expressed in human Tregs. HDAC9 deficiency: 1) increases Foxp3 expression (and other Treg markers), 2) increases Foxp3 and histone 3 acetylation, 3) increases Foxp3 DNA binding, 4) increases Treg numbers, 5) increases suppressive activity in vitro and in vivo, and 6) ameliorates murine colitis. Tregs which are deficient in HDAC9 induce permanent tolerance of fully mismatched cardiac allografts. In addition, HDAC9 inhibitors maybe useful for treatment of diseases and disorders associated with abnormal cell proliferation, differentiation and survival, e.g. breast and prostate tumors.
Preliminary data shows that targeting HDAC7, a class IIa histone deacetylase, enhances Treg suppression in vitro and in vivo. HDAC7 enhances FOXP3+ Treg function and induces long-term allograft survival.
Inhibition of HDAC6, a class IIb HDAC, has been shown to increase Treg suppressive function in vitro along with increased expression of FOXP3 protein and Treg associated genes including CTLA, IL-10, TNR18. HDAC6 inhibition in vivo decreased severity of colitis in the dextran sodium sulphate-induced colitis model and the CD4+CD62Lhigh adoptive transfer model of colitis. In addition, inhibition of HDAC6 with a subtherapeutic dose of rapamycin led to prolonged cardiac allograft survival.
Based on the above evidence, an orally available small molecule selective inhibitor of Class II HDAC activity (more specifically HDAC9 or HDAC7 or HDAC6) is expected to modulate autoimmune diseases through expansion and enhancement of Treg activity.
Inhibition of other Class II HDAC's for example HDAC4 and 5 impair myogenesis by modulating the stability and activity of HDAC-MEF2 complexes and maybe potentially useful for the treatment of muscle and heart diseases including cardiac hypertrophy and heart failure. Also, inhibition of Class II HDAC activity, represents a novel approach for disrupting or intervening in cell cycle regulation.
Class II HDAC inhibitors have therapeutic potential in the study and/or treatment of diseases or conditions ameliorated by modulating HDAC activity (in particular, cell proliferative diseases (such as cancer), diabetes (type I and/or type II diabetes), inflammation, cardiac disease, obesity, stroke, epilepsy, depression, immunological disease or viral or fungal infection.
Many HDAC inhibitors, however, inhibit all HDAC isoforms. It would be advantageous to identify HDAC inhibitors that inhibited one or more but not all HDAC isoforms.